Two of the most desirable features of a cell for electrochemical synthesis are a high electrochemical efficiency and a low power consumption per unit of product.
High electrochemical efficiency is achieved if the concentration of electroactive species adjacent to the electrode is high. In some processes this occurs naturally, for example if the concentration of electroactive species in the solution is high. This type of reaction is described as being independent of mass transport. In many other electrochemical reactions, however, the electroactive species is in low concentration or is in competition with other species in solution. This type of reaction is mass transport limited and high electrochemical efficiency may be achieved in a cell in which the mass transport is enhanced.
The current efficiency is determined by the relative rates at which the various ions present are discharged at the electrodes. One method of increasing current density which has been proposed and is well documented in the scientific literature (for Example J. Applied Electrochem 7, 473(1977); Desalination 13, 171(1973); Electro Chemica Acta 22, 1155(1977)) is the use of a so-called "turbulence promoter" usually in the form of a mesh of plastic or some other inert material adjacent one electrode of the cell and spaced from its facing electrode.
The spacing of the electrodes must be greater than the thickness of the turbulence promoter because otherwise, using the turbulence promoters described, there would be no residual flow path through the cell. Also there is a difficulty in practice in reducing the electrode gap in any cell employing flat plate electrodes and requiring liquid circulation in that the spacing of such electrodes dictates the thickness of the frames in which they are mounted and which separate the anode and cathode electrodes. At low separations the frames become too thin to allow adequate liquid flow channels to be formed in them for circulating the electrolyte through the cell over the turbulence promoter if one is present.
Low power consumption is achieved by reducing the total potential of the cell. This may be considered as being made up of three components: the anode potential, the cathode potential and the potential drop in the intervening solution. It is not generally possible to reduce the electrode potential as its value determines the electrochemical process occurring on its surface. In order to reduce the overall potential one generally attempts to reduce the potential drop in the solution. In highly conducting solutions this will be small, but in poorly conducting solutions it will be significant and will certainly be the major component of the total cell potential. Many cells have been designed to overcome these problems in a variety of ways.
One of these is known as the Capilliary gap cell (Chem.Ing.Tech. 41, 943 (1969), Fr. Pat. No. 1,476,162). This device consists of a stack of circular electrodes each with a hole cut out of the centre (rather like a gramophone record). Electrolyte is fed down a central pipe which is slotted to allow electrolyte to flow out radially between adjacent electrodes. The electrodes are separated by narrow shims of non-conducting material (see diagram). In this way very small inter-electrode gaps are possible. The disadvantages of the cell are that it is difficult to engineer and that separate anolyte and catholyte streams are not possible. In addition, a bipolar unit is only possible under certain limited values of conductivity.
In the fluidised bed cell, electrodes are separated by a mass of fluidised non-conducting particles which enhance mass transport but dictate a minimum inter-electrode gap of at least 10 mm to achieve satisfactory fluidisation. These cells are accordingly only suitable for relatively conductive electrolytes.
A rotary cylinder cell is described in British Specification No. 1505736. In this cell good mass transport is achieved by having one of the electrodes in the form of a rotating cylinder. The cell is useful for producing powders but has the disadvantage that it is difficult to engineer and maintain and a significant amount of power is used to rotate the cylinder.
In the pump cell described by R. E. W. Jansson in J. Appl. Electrochem (1977)437, which is similar in concept to the capilliary gap cell, the major difference being that alternate disc shaped electrodes are rotated relative to their static neighbours, good mass transport is again achieved, but a divided cell is not possible and the engineering is complex.
The features of all the cells described above are presented below in tabular form.
__________________________________________________________________________ Good Small inter- Divided cell Absence of Easy con- mass electrode may be ancilliary struction & transport gap constructed power maintenance __________________________________________________________________________ Capilliary -- -- -- Gap cell Rotating -- -- Cylinder Cell Pump Cell -- -- -- Fluidised -- bed cell Fixed -- turbulence promoter cell __________________________________________________________________________